The principle of a vibrating fork level sensor is simple. A tuning fork is caused to vibrate at its natural frequency by a piezoelectric crystal assembly and associated electronic circuit. As is well known, the natural frequency changes when the fork is immersed in liquid. Depending on how the sensor is configured, a change in the natural frequency beyond a threshold serves as an indication that the fork assembly is either become immersed in a fluid or, having been immersed in a fluid, is now no longer immersed.
Instruments of this type are typically calibrated when the fork assembly is dry. Once the dry frequency has been determined, the thresholds at which the instrument is assumed to be wet, or is faulty, can be established with reference to the resonant frequency, and stored in the operating electronics. When in service, if the fork assembly is dry, the operating frequency is compared with a low threshold. If the frequency falls below that threshold, the fork assembly is assumed to be wet. If the instrument is wet, the operating frequency is compared with a different, upper, threshold and, when this threshold is exceeded, the fork assembly is assumed to be dry.
It is recognized that the calibration frequency can change due to changes in the condition of the fork assembly. For example, the fork assembly can acquire a coating which increases the mass of the fork assembly and reduces its resonant frequency. The fork assembly can also suffer from corrosion. In this event, the mass of the fork assembly decreases and the resonant frequency increases.
These alterations in the fork assembly can reach a point at which false indications of wet or dry can be given.
It is an object of the invention to provide a form of vibrating element apparatus which will address the problems set forth above; or which will at least provide a novel and useful alternative.